In the process of electrophotography an image is recorded in the form of an electrostatic latent image on a photosensitive member. The latent image is then rendered optically visible by application of electroscopic marking particles commonly referred to as toner. The toner-based image may be affixed to the photosensitive member or may be transferred to another substrate and affixed thereto. The toner is commonly fixed or fused to the substrate by a combination of heat and pressure. That is, the temperature of the toner is elevated to a point at which elements of the toner become tacky such that these elements flow into fiber or pores or otherwise flow along the substrate surface. Thereafter, as the toner material cools, it solidifies and becomes bonded firmly to the substrate.
A conventional approach to heat and pressure fusing of electrostatic images on a support substrate, such as paper, involves passing the substrate with the toner images formed thereon between a pair of roller members at least one of which is heated. The heated member is commonly referred to as the fuser roller. Since the toner image is tackified by the heat, part of the intended image carried by the substrate surface may adhere to a portion of the fuser roller surface. As a second substrate surface is brought into contact with that same portion of the roller surface to receive a second intended image, the portion of the tackified first intended image that was partially transferred to the roller surface transfers to the second substrate surface.
During the same process, part of the tackified second image intended for the second substrate surface may also adhere to the heated roller such that an unintended image transfer again occurs. That is, with a portion of the tackified second intended image having been transferred to the roller surface, there is a partial transfer of the second image from a portion of the roller surface to a third substrate surface when a third image is being formed on the third substrate. Also, during revolution of the various roller members without a substrate coming into contact with the fuser roller, tackified toner which becomes affixed to the fuser roller may transfer to another roller, e.g., the pressure roller. Generally, such occurrences are referred to as “offset”.
Particles of toner are offset, i.e., transferred, to the fuser roller for a variety of reasons, including insufficient heating, surface imperfections on the fuser roller or insufficient electrostatic forces to hold the toner particles against the substrate. Several solutions have been provided to mitigate this problem. Typically, the surface of the fuser roller is coated with a low-surface energy release agent fluid, such as silicone oil. Such release agent fluids are transferred to the fuser roller from a release agent (oil) sump, via a wick apparatus or a roller assembly. In the roller assembly, one or more roller surfaces are wet with the release agent and, through rolling action, the release agent is transferred to the fuser roller. See, for example, U.S. Pat. Nos. 6,075,966 and 6,112,045 each now incorporated herein by reference. It is desirable that such roller assemblies, referred to as oiler systems, pass a controlled and consistent amount of oil, i.e., release agent, to the fuser roller.
Despite numerous modifications and improvements made to such oiler systems, undesirable characteristics persist. For paper substrates, it is common to transfer some oil from the fuser roller to the sheet, e.g., four to eight mg per sheet of A4 paper. However, in multi-sheet printing operations it is not uncommon for the oil transfer rate to begin at three to four times the desired rate and to substantially decline after the first ten to twenty sheets are processed. This surge of release agent may be attributed to several factors. Residual release agent fluid is commonly left on the fuser roller surface from prior reproduction runs. The amount of such release agent fluid depends in part on the split ratio between rollers. With a simple 50 percent split in release agent fluid volume between rollers, the residual release agent fluid on the fuser roller can rise to four times the steady state rate.
In addition, if the oiler system remains idle for a significant time interval, e.g., five to ten minutes, some release agent fluid will migrate from the sump by capillary forces. With this accumulation in place, when the oiler system is next engaged a surge of release agent fluid, e.g., tens of mgs, will be transferred to the fuser roller and ultimately to the substrate.
Another factor affecting the volume of release agent fluid transferred is the viscosity of the release agent fluid, which, as is well known, varies substantially with temperature fluctuations. Thus, in systems which require thermal fusing of the toner, temperature variations are to be expected and such variations will have a temporal influence on viscosity. Predictably, the temperature of the release agent fluid is relatively low at the beginning of a reproduction run and increases as each sheet is processed during the run. While it is somewhat difficult to quantify the viscosity variation, limited tests indicate that normal heating can alter the viscosity to the point where, if other variables remain constant, the release agent fluid transfer rate may at least double.
The release agent fluid transfer rate is also affected by uncontrollable variations in roller speeds; particularly, in a roller assembly oiler system, the speed of a metering roller which is driven by a donor roller. When there is too much oil on the adjoining surfaces or there is excessive drag force caused by the wick of a wick apparatus, substantial slippage occurs. In turn, this results in slower movement of the metering roller. As the metering roller speed decreases, the amount of release agent fluid transferred to the donor roller also decreases. It should also be noted that, when there is a speed differential between the rollers, a drag force may persist which force can accelerate wear of the fuser roller.
The aforementioned variables are believed to result in non-uniform and somewhat unpredictable release agent fluid transfer rates. Further, notwithstanding these uncontrollable variations, such oiler systems are designed according to fixed release agent fluid transfer rates and do not have means for adjusting the release agent fluid transfer rates.
It is desirable to provide methods and systems, which improve the consistency and uniformity of transferring the release agent fluid. Such improvements would result in more satisfactory image reproduction and lower maintenance of associated equipment. It is also desirable to control the rate of release agent fluid transfer to the fuser roller. In conventional oiler system designs, one or more operating parameters may be selected to control the transfer rate, but because these are fixed for each design, there is a need for a system wherein the release agent fluid transfer rate is adjustable in order to further improve the quality of image reproduction.